Looking it over quickly, this is the value you are looking for, and likely what the remzibi 3DR calls "heading". (It would actually be true course if provided by a GPS). The important issue is the format. It looks like a string 5 characters long is required.
So let's say you have a WORD variable MyHeading that contains a value between 0-359. To output this as it appears in the NMEA sentence use:
SEROUT bob, baud, [DEC3 MyHeading , ".0"]
You can see I added the decimal point and zero to fill out the five characters.
HTH
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